M12-T01: Extract shared retire_queue across allocdb-core, quota-core, and reservation-core

[skel84]

Summary

Extract retire_queue as the first tiny internal shared runtime module.

Scope

• move the identical retire_queue implementation behind one shared internal boundary
• update all three engines to consume the shared module
• keep engine semantics unchanged

Acceptance

• all three engines use the shared retire_queue
• no behavior changes
• all existing tests remain green

Canonical docs

• docs/runtime-extraction-roadmap.md
• docs/reservation-runtime-seam-evaluation.md

Blocked by

• M12: Extract the first internal shared runtime substrate from the three-engine family #120